With a light weight (5 to 8 lbs) in both hands, press from your chest straight up to locked arms above. Then come down and touch the weight to your left foot. Up to the sky again with a press. Now down to your right foot. Do 10 reps on each side. Rest. Then do 8 to each side. Rest. Then 6 to each side.
If full push-ups are tough for you, these are the answer. Place your feet a body length from the desk or table. Put your hands on the desk right in front of your shoulders. Lower yourself down, body one flat plank, until your chest touches the desk. Do 12. Then 10. Then 8. Then 6.